The present invention relates generally to the production of industrial usable gas in a two-stage gas producer and, more particularly, to a method for cleaning gas produced from solid carbonaceous material in a two-stage gas producer to provide a clean industrial usable gas.
In a two-stage gas producer process, second stage gas, also known as top gas, and first stage gas, also known as bottom gas, are produced. Although it is highly desirable to collect and use the first and second stage gases from the gas producer, the collection must be performed in a manner that not only yields a clean industrial usable gas, but also does it in an environmentally and economically sound manner.
The second stage gas contains a mist of small particle size oil droplets and solid particulates, primarily coal fines that are either introduced with the coal feed or made in the gas producer. The first stage gas does not contain oil mist, but it does contain finely divided solid particulates. The particulates are produced in the lower section of the gas producer from the break-up of char caused by the motion of the solids. Therefore, the oil mist and particulates should be removed from the first and second stage gases to produce a clean industrial usable gas.
Various methods have been attempted to remove oil mist and solid particulates from the gases produced by a two stage gas producer. In a hot raw gas technique, the first and second stage gases are passed through separate cyclones to remove oil mist and particulates. After the cyclone treatments, the two gas streams are combined to produce a gas product. This method, however, inherently has a low efficiency for removal of particles having diameters less than 50 microns. As a result, only a part of the oil mist and particulate contamination are removed from the gas product and, hence, the resulting gas is not a clean industrial usable gas.
In a treatment known as hot detarred gas, the second stage gas is passed through an electrostatic precipitator to remove oil mist and particulates. The first stage gas is separately passed through a cyclone to remove particulates. The resulting gas streams are then combined to produce a gas product. Unfortunately, the combined gas still contains significant levels of particulate contamination because of the low efficiency of the cyclone for removal of particulates from the first stage gas. This particulate contamination renders the resulting gas unacceptable for industrial uses that require a clean gas.
In another method, known as the cold clean gas method, the second stage gas is passed through an electrostatic precipitator, while the first stage gas is separately passed through a cyclone and a heat exchanger to reduce the temperature of the first stage gas to about 250.degree. F. The first stage gas is then mixed with the second stage gas, and the gas mixture is subsequently scrubbed with an aqueous liquor in an indirectly cooled scrubbing tower in which the temperature of the gas mixture is reduced to about 100.degree. F. The gas from the scrubbing tower passes through an electrostatic precipitator to yield a gas product.
A key disadvantage of this method, however, is that the aqueous liquor produced in the scrubbing tower contains dissolved organic compounds, such as phenols and pyridines, as well as ammonia and hydrogen sulfide. This contaminated liquor must be either treated extensively before it is discharged from the gas producer facility or incinerated to avoid an adverse impact on the environment. Additionally, in the cold clean gas treatment method, substantial amounts of the heat of the first and second stage gases are lost because of the necessity of cooling the gas mixture to a relatively low temperature to achieve effective scrubbing with the aqueous liquor in the scrubbing tower. As a result, the overall efficiency and economy of the process is substantially reduced.
In summary, all of the previous gas cleaning techniques have significant disadvantages that make them unacceptable for widespread industrial use. None of the previous cleaning techniques provides a clean industrial usable gas, while also satisfying the requisite environmental and economic constraints.